Blood pressure measuring device with a cuff of two openable concave shell parts

ABSTRACT

Traditional cuffs for measuring blood pressure use an air chamber enclosed in a non-stretchable fabric to occlude an artery in a limb when supplied with pressurised air. A stethoscope used on the limb is used to monitor blood flow. Application of the cuff is inconvenient and correct placement of the stethoscope chestpiece requires skill. According to the invention blood pressure measurement is facilitated by having the air chamber enclosed in a pre-formed shell-like structure being flexible around the limb and stiff along the limb and by using a linear array of microphones to detect the blood flow noises, the best signal from one of the microphones being automatically selected. Furthermore, the invention provides a facility for correcting the reading in dependence of the amount of wrap of the limb.

The invention concerns apparatus for measuring blood pressure,comprising a generally tubular constrictable sleeve or cuff for a limbof a person, a source for fluid pressure, means for measuring staticpressure, and microphone means arranged in proximity to an artery.

Modern blood pressure measurements have long traditions and fall intotwo distinct types. Both the auscultatory and the oscillometric methoduse the constriction of an artery to such a degree that blood flow isstopped and then allowed to flow while a signal derived from the bloodpressure is monitored. The constriction occurs by means of a cuffsurrounding a limb (in most cases an upper arm or a wrist). The cuff hasa non-stretchable fabric on the outside enclosing an elongate bladdersurrounding a large part of the limb periphery. The bladder ispressurised by means of air, and the air pressure is monitored. TheKorotkoff method depends on listening to sounds in the artery downstreamof the constriction as blood begins to flow, and to read the pressurewhen certain sounds related to the heartbeat are heard and again whensounds begin to disappear. Traditionally, the listening has occurred bymeans of a stethoscope, the chestpiece of which is held against the skinin proximity to the artery downstream from the occlusion, frequentlysupported against the edge of the cuff.

The above process of measuring blood pressure is perceived as a slowprocess and one which requires skill. This is due to the manipulation,requiring two hands, involved in fitting the cuff, and the need forprecise placement of the stethoscope chestpiece. The pumping and releaseof air are perceived as the least time consuming, particularly becausethey pertain to the actual measurement. Modem measurement methods useautomatic pumping and release and electronic microphone pickup of theKorotkoff sounds and possibly some signal processing aids indistinguishing between the various types of sound.

U.S. Pat. No. 4,337,778 describes an attempt to reduce the entanglementof air tube and microphone lead in connection with the wrapping andunwrapping of the cuff, in that the Korotkoff sounds are picked up bymeans of a microphone inside the inflatable bladder. However, the patentdoes not attempt to solve the fundamental time-consuming problem of thewrapping and unwrapping for the single-handed individual.

In U.S. Pat. No. 5,560,365 it is described how the provision of apartially stretchable cuff may reduce friction noises in case the bloodpressure measurement is performed on a non-stationary limb. It improvesthe signal to be analysed, but it does not solve the problem of fittingthe cuff.

In U.S. Pat. No. 4,248,242 it is described how a blood pressureapparatus may be semi-automated by means of a sequential switch and handpump arranged on apparatus integrated with the cuff The cuff itselfstill has to be threaded on the arm. A sensor is provided for picking-upthe Korotkoff sounds but there is no indication of the manner in whichit is fitted to the apparatus.

U.S. Pat. No. 4,790,325 relates to an automatic blood pressure recorder,in which the patient is required to sit and place his arm in a solidfixture or jig integrated into an armrest and containing inflatableelements, said fixture being closed automatically by an equally solidclamp. This makes the equipment very stationary and ill suited forpatients unable to sit upright in a chair-like construction.

It is hence an object of the invention to provide a cuff-and-microphonecombination that is able to provide consistent good acoustic couplingand signal processing to obtain dependable artery sound signals. It is afurther object of the invention to provide a cuff structure that isadaptable to a wider range of biometric measures of a limb than knownapparatus. It is a still further object of the invention to provide anindication of a mismatch in case the biometric measurements servicedreliably by the cuff fall outside the limits of its adaptability and topropose a correction commensurate with the mismatch.

According to the invention the above deficiencies are avoided and theadvantages obtained, in that the cuff is at least partly enclosed in twoessentially concave shell parts displaying a stiffness along the limb,said shell part being openable against a restoring force, and in that alinear array of microphone elements is disposed on a universal jointtype support in one shell part essentially perpendicular to thelongitudinal axis of such shell part and near the lower end. The term“concave” refers to the fact that the shell parts may be straightaxially along the arm, whereas they intendedly curve in the directionperpendicular to the axial direction along the arm. The term “stiffness”refers to the fact that it is not intended to give the shell a curvaturein the axial direction that deviates from any curvature it may have beengiven during manufacture. A universal joint type support for a lineararray perpendicular to an axis is characterised by permitting movementin a plane perpendicular to said axis as well as rotation about an axisalong the linear array. There is a distinct advantage to using a stiffshell for enclosing the inflatable cuff, rather than the traditionalwoven strap, because the forces between the cuff and the limb (upperarm) are more evenly distributed and so facilitate a stable andrepeatable occlusion of the artery. This is important both for theauscultatory and for the oscillometric method. In case of theauscultatory method the rotational precision required in fitting theimplement is much reduced by providing several microphones incombination with the stiff shell, because it will at all times bepossible to find the microphone which provides the clearest signal.There is a particular advantage to using such an array of microphoneswhen the inflated cuff is retained by a stiff shell, because therepeatability of fitting the apparatus and of the readings are hugelyincreased. The implement may be entirely supported by a limb, i.e.without attachment to constructive elements carried by e.g. an arm rest.

According to an advantageous embodiment signal selection means of thediversity reception type are used to select the microphone that providesthe best signal-to-noise ratio. Rather than averaging the output of thelinear array of microphones it is much more efficient to select themicrophone which at the same time receives the strongest signal but alsothe least amount of extraneous noise.

According to a further advantageous embodiment the microphone signal isamplified and made available to an electroacoustic converter forenabling listening to the signal. This means that an examining physicianmay demonstrate to others, including the patient, the character of theKorotkoff sounds directly from the apparatus of the invention, ratherthan from a separate stethoscope.

According to a further advantageous embodiment the signal is output viaa built-in speaker in the apparatus. This makes the apparatus completelyself-contained.

According to a further advantageous embodiment the signal is output viaa wireless link to a receiver connected to earpieces carried by anauscultating physician. Such a receiver/earpiece combination maytypically be a part of an electronic stethoscope already carried by thephysician and other medical staff surrounding the person whose bloodpressure is measured by the present apparatus.

A further advantageous embodiment of the invention is particular in thatit comprises signal processing means for combining information derivedfrom measurements of slowly varying static pressures with informationfrom said microphone means in order to obtain a numerical value for ablood pressure. This would typically include cycling the static pressureand obtaining a sound signal in dependence thereof, the frequencycontent of said signal determining the type of Korotkoff sound detected,and sampling said static pressure and combining with frequency contentsignatures sampled essentially simultaneously therewith will providenumerical information of the pressures required to obtain specificKorotkoff type sounds.

According to a further advantageous embodiment an inelastic strapattached to one shell part is provided to close the gap between theshell parts. The use of a strap is known per se from traditional cuffs,however, according to the invention its action is more consistentbecause it attaches the shell parts.

According to a further advantageous embodiment the strap is providedwith means locking to the other shell part in conjunction with theoverlapping of said strap and said other shell part. Such means would beof a quick-release type.

According to a further advantageous embodiment the amount of overlapbetween the strap and the shell is used as a circumference measure forautomatically correcting the reading of blood pressure. It is well knownthat for a constant circumference of the cuff fitted snugly to a limb,the precision to which the systolic and diastolic pressures are givendepends on the axial dimension of the cuff. The length of the implementaccording to the present invention being constant and precise, due tothe stiffness of the shell, knowledge of the circumference enables asuitable correction to be applied to the reading. The absolute value ofthe circumference is equal to the circumference of the cuff plus thecontribution of the flap. The correction is performed during the signalprocessing in dependence of the overlap signal from the strap. Theskilled person will choose any of several technologies available forthis kind of relative position measurement. Hence the need for severalsizes of cuffs known from traditional blood pressure measuring setups isreduced.

According to a further advantageous embodiment the overlap is measuredcapacitively between an electrode or a plurality of electrodes fixed tothe cuff and an electrode or a plurality of electrodes fixed to thestrap. A capacitive detection and capacitive transmission of measurementdata is well suited for a blood pressure measuring implement, which mayuse disposable parts, because expensive and time-consumingplug-and-socket connections are avoided.

According to an advantageous embodiment the shells are fitted on a hingeconnected to handle parts operable by one hand. This in effect meansthat correct placement of the implement is possible using one hand only,whereby a person may use the implement on himself.

According to a further advantageous embodiment in addition to astiffness in the longitudinal direction the shell structure displays aresilience in the circumferential direction. In this manner, the shellwill adapt still closer to the limb when it is gripping it.

According to a further advantageous embodiment the hinge is a continuousresilient part joining the shell parts. It is important that the shellsfits closely to the tissue when the implement is fitted, and the handlesact against the force of such a resilient part.

According to a further advantageous embodiment the shell parts areintegral with the hinge part, forming one continuous sheet of material.This means that the implement may be manufactured in one piece which isgiven its proper shape during manufacture. An example of this continuoussheet structure for the shell parts may be envisaged, in which theresilience in the circumferential direction is actually an extreme“limpness”. Integral shell parts in the form of a woven sheet havingflexible but generally unstretchable strings as the warp and a number ofparallel, stiff strips as the weft would be oriented on a limb so thatthe weft direction would be parallel to the axis of the limb.

According to a further advantageous embodiment the continuous sheet ofmaterial assumes a generally frusto-conical shape in its closed state.This may require a “pre-distortion” of the shape of the implement andwould be most practical in connection with well-developed muscles in theupper arm. However, it has been determined that in connection with theshell-type construction according to the invention the frusto-conicalshape of the implement has a larger range of adaptation to biometricalmeasurements than a cylindrical type.

According to a further advantageous embodiment mechanical actuatingmeans fitted in proximity to the hinge part compress one shell parttowards the other during measurement. In case the implement is to beused by persons having too little strength in the actuating hand, theopening of the shell parts by means of the handles may be made torequire only a small force. However, in this case some mechanicalassistance is required to compress the two shells towards each otherprior to fitting a strap and measurement.

According to a further advantageous embodiment the mechanical actuatingmeans consist of an air cylinder and levers. Air under pressure isavailable for inflating the air chambers of the cuff, and hence asuitable actuating means for the shell parts would be an air cylinder oranother closed shape-changing vessel.

According to a further advantageous embodiment the mechanical actuatingmeans consist of strings fitted near the inner side of each shell partand disposed perpendicular to the longitudinal axis of such shell part.This will give an action similar to the action of the tendons in a handwhen closing it into a fist. This type of actuation is more adapted toelectric power, because the strings may be tightened by winding them ona rotating shaft.

According to an embodiment of the invention an inflatable cuff forms aninner lining to the shell parts, providing an inflatable main airchamber. With the secure support of the shell parts it is possible toobtain a secure and repeatable inflation, without any distortion of theair chamber that could lead to difficulties in occluding the bloodvessels.

According to a further advantageous embodiment the strap is providedwith air chambers disposed essentially perpendicular to the orientationof the shells and communicating with the main air chamber. This ineffect means that an inflatable structure surrounds the whole arm, againproviding improved repeatability in inflation.

According to a further advantageous embodiment the universal joint isemulated by a foam pad. This is an efficient manner to provide a supportfor the microphone array or bridge, both from a manufacturing viewpointand because the resilience of the foam pad may be adjusted incorrespondence to the length of the bridge, thickness of the microphoneelements, etc. A foam pad will permit certain deviations from theperpendicular position of the microphone bridge, i.e. permit a closerfit to the skin when so required.

According to a further advantageous embodiment the universal joint isemulated by means of a separate air chamber fitted between the cuff andthe microphone array. This means that the contact pressure of themicrophone bridge may be adjusted during measurement, if required.

One use of the apparatus is for an upper arm, in which the possibilityof using only one hand for fitting it makes it entirely practical for apatient to monitor blood pressure without assistance from medicalpersonnel.

A further use of the apparatus is for a leg, in particular in caseswhere injuries to the upper torso prevent the measurements on arms. Thedimensions of the implement in question must generally be larger thanfor an arm, however even in this case, the precise location of an arteryis not required, as the advantages of using a microphone array incombination with a shell structure would still manifest themselves.

The invention will be more fully described in the following withreference to the drawing, in which:

FIG. 1 shows a first cross section through one embodiment of theinvention fitted to a limb,

FIG. 2 shows a second cross section, further along the limb,

FIG. 3 shows a third cross section, still further along the limb,

FIG. 4 shows another embodiment of the invention, seen from the inside,

FIG. 5 shows signal processing means for selecting a signal with a goodsignal-to-noise ratio,

FIG. 6 shows one embodiment of a capacitive overlap detector, and

FIG. 7 shows the electrical equivalent circuit of such a detector.

In FIG. 1 there is shown a cross section through a limb around which theinventive implement is fitted. Purely by way of example this is to beconsidered as an upper left arm extended horizontally forwards withrespect to a vertical torso. The arm 1, the bone 2, an artery 3 and avein 4 are indicated. The arm 1 is enclosed in a shell-like structureconsisting of an upper part 5 and a lower part 6, connected by a part 7.

The parts are made of a material which is pliable around the arm butstiff in its longitudinal direction. To the inside of parts 5, 6, 7 isfitted an inflatable structure 8 which is able to compress the tissue ofthe arm 1 around the bone 2, provided the shell-like structure does notincrease its outer dimensions.

In FIG. 2 is shown how the outer dimensions of the shell-like structureare maintained. This is obtained in that a strap or flap 9 is connectedbetween the upper part 5 of the shell-like structure and thecorresponding lower part 6. The flap is made in a material which doesnot extend when put under tension, and so the circumference of theinventive implement is constant. When the inflatable structure 8 isinflated, the implement tends towards a shape which provides the largestarea for a given circumference which is a circle, and the tissue in thearm 1 is compressed to a degree which may occlude the blood vessels 3and 4. The implement is fitted with two handle-like structures 10 and 11which when brought towards each other will increase the distance betweenthe parts 5 and 6, provided the flap 9 has not been connected. Aresilient member 12 is acting on the handle structures 10 and 11,attempting to close the shell-like structure 5, 6.

In practical use one hand may act on the handles 10, 11 to open theshell parts 5, 6 in order to fit the implement on the arm 1; in theexample above this would be from the right towards the left. The handles10, 11 are released, the resilient member 12 expands in closing theshell parts 5, 6, and a loose end 13 hanging down from the flap 9connected to the upper part 5 is gripped from below with the same handthat released the handles and is brought to the right with a pullingmotion. The flap 9 attaches itself to the lower part 6 by releasablemeans, such as the fastener type described in U.S. Pat. No. 2,717,437.The inflatable structure 8 is provided by air under pressure, and theblood vessels are occluded when sufficient pressure has built up.

In FIG. 3 the downstream (looking at the flow in an artery) end of theimplement is shown in cross section. This end carries a set ofmicrophones for picking-up signals from the flow in the artery 3. Threemicrophones 14, 15, and 16 are fitted onto a fixture 17 which is mountedon a universal joint type of bearing 18 in order that the fixture mayfit closely to the skin in proximity to the artery 3. In a preferredembodiment the universal joint is emulated by a foam pad. One microphone16 is shown as being closest to the artery, and the signal from thismicrophone will have the best signal-to-noise ratio of the three. Thisis in contrast to a construction embodying an elongate microphone, suchas that described in U.S. Pat. No. 4,202,348. While this patent doesdescribe how it is ensured that a signal from an artery is picked up fora range of rotational orientations of the cuff holding the microphone,said construction equally collects various noise signals.

The universal joint type bearing 18 may equally be a small air cushion,not necessarily connected to the main inflatable structure.

In FIG. 4 is shown a different embodiment of the invention, but forclarity it is shown in a form that it would never assume as a finishedproduct. In this case the two shells 50, 60 and the hinge 70 are made ofone sheet of material in which the stiffness is dependent on thedirection. It is considerably stiffer in its longitudinal direction thancrosswise (around a limb). Such a material may consist of a number ofparallel linear structures of increased thickness joined by areas ofreduced thickness or may be obtained by corrugation. The upper and lower“shell” parts are pre-formed in a concave shape towards each other, andas in FIG. 2 actuating “handle” parts are able to separate the two“shell” parts, the “hinge” part being equally pre-formed duringmanufacture. The cross section of this embodiment is similar to FIG. 2,however the general outline of the sheet material is such that it willconform to a frusto-conical shape when the implement is applied.

In FIG. 4 the “shell” and “hinge” parts 50, 60, 70 are shown “flattened”to the image plane. Furthermore is shown the main air chamber 80 liningthe sheet of material as well as supplementary air chambers 81, 82, 83,84 lining the strap or flap 90 which is used to close the implement in amanner described in connection with FIG. 2. The communicating airchambers may advantageously be made in a flexible weldable poly-urethanesheet material. The position of handles 100 and 110 is indicated bydot-dash lines. It will be obvious to the skilled person that theorientation of the flap 90, i.e. whether the closing movement is upwardsor downwards is immaterial to the construction, although it may beimportant to the user. The closing occurs by means of fasteners 91co-operating with corresponding fasteners on the outside of the part 60of the implement. The bridge 170 carrying the microphones 140 is shownadjacent to the air chamber 80.

Upper arms being very different in a population, there may be a need fordifferent sizes of implement, however the implement according to theinvention will adapt to many biometrical dimensions, the basicfrusto-conical shape being adaptable to create even a cylindrical cuffif required. However, the provision of supplementary air chambers in theflap itself enables a larger range of upper arms to be measured with oneand the same implement, than for known cuffs.

The strap 90 and its fastening flap 91 for joining to the outside of theshell 60 are provided with extension measuring means 92, 93 which areable to provide a signal indicating the amount of overlap of the strapwhen fitted to the limb. The overlap is used as a measure representingthe degree of encirclement of the arm by the inflatable part. It hasbeen determined that for reliable readings, the inflatable part mustencircle ca. 80% of the arm. Furthermore, the width of the inflatablepart must be in the neighbourhood of 40% of the circumference of thearm.

One embodiment of an overlap detector is described in conjunction withFIG. 6. This means that a signal related to the circumference of thelimb is readily available with sufficient precision for use incorrecting the reading of systolic and diastolic pressures. Thiscorrection may be performed automatically by the implement in the DSPenvironment assisting the measurement. Alternatively, the DSP functionsmay alert the user to the fact that the range for normal measurement hasbeen exceeded, it may propose a correction, or provide the rawmeasurement data, leaving the decision of how to handle the mismatch tothe examining physician.

In FIG. 5 is shown the principle for selecting the microphone thatprovides the most significant signal related to the flow in the partlyoccluded artery 3. A number of microphones 140 are connected to a set ofpre-amplifiers 20, and each amplified signal is brought to a processingunit 21, which performs digital signal processing by emulating functionscomprising high frequency pre-emphasis means, strobing means forselecting each microphone signal, level detection means for each signal,storage means corresponding to each signal for the level of highfrequency present, comparator means for comparing the level of a newlystrobed signal with those stored, and selector means for taking thesignal fulfilling the set criteria to a unit 22, in which it is madeavailable to the ear and to a visual indication. In this conjunctionelectronic transmission of the signal for further processing may alsooccur. The unit 23 is a timing and synchronising unit which aids in theselection according to the set criteria by linking the measurements inunit 21 to other measurable quantities having a time function.

In FIG. 6 is shown one embodiment of a capacitive overlap detector. Anumber of fixed capacitor plates P1, P2, . . . Pn are disposed at oneend of the cuff in a row in the longitudinal (peripheral) direction ofthe cuff. The capacitor plates are connected to taps in a seriesconnection of resistors R1, R2, . . . Rn supplied with a high frequencycurrent provided via a capacitive coupling consisting of furtherparallel plates C1, C2 to a suitable part of the shell. The groundconnection to the other end of the series connection of resistors isshown as capacitor plates C5, C6. The row of capacitor plates cooperateswith a sliding capacitor plate D disposed at the other end of the cuffand brought in parallel proximity during the closing of the strap. Thesliding capacitor plate D is connected to a predetermined part of theshell via coupling capacitor plates C3, C4. This construction functionsas a capacitively coupled potentiometer which provides a sufficientlylinear output voltage V in dependence of the placement of the slidingcapacitor plate D, and it falls within the tasks of the skilled personto design the precise configuration for any practical strap, itsthickness and dielectric constant. The voltage may be determined as thereal part of the complex voltage V when the impedance Z of theresistor-capacitor network is fed by a current I. The cuff and strap (asopposed to the shell) are disposable and/or autoclaveable parts, andhence all signal coupling is capacitive. Preferably all capacitor platesare made in metal foil, however the high impedances involved makes theuse of metallised plastic foil equally useful.

Signal processing means convert the output voltage to a value which isprovided to the data processing means in order that a suitablecorrection may be obtained, preferably by accessing a table ofcorrections. If the circumference is outside the limits for theparticular cuff, a warning can be given. It is also possible to suggesta correction for the measured blood pressure when the cuffwidth/circumference ratio differs from 0.4 (40%).

The equivalent circuit of the preferred embodiment of the system fordetermining degree of overlap is shown in FIG. 7. Provided thatZ_(load)is relatively high the current generator forces a constantcurrent to flow in the loop, (Ci1 Rvar Co Ci2). The loop capacitance,the imaginary part of Z_(loop), in the couplings are dependent of theformed geometry, area and distance between the plates. The loopresistance, the real part of Z_(loop), is only dependent on the sliderposition, the overlap of the cuff ends, and thereby related tocircumference. As the current is constant, the voltage across thecurrent generator is proportional to the loop impedance. By determiningthe real part of the voltage, the part in phase with the current, thecircumference can be expressed. If the sampling frequency is much higherthan the frequency of the current, the calculation can be done by thebuilt in micro-processor. However, to ensure that the imaginaryimpedance is on a reasonable level, compared to the real impedance, arelatively high frequency for the current is desirable. In order to keepthe sampling frequency down the real part is extracted by by analogquadrature detection. The total circumference is expressed as:circumference=Xo+f(real(V))

The foregoing description of the specific embodiments will so fullyreveal the general nature of the present invention that others skilledin the art can, by applying current knowledge, readily modify or adaptfor various applications such specific embodiments without undueexperimentation and without departing from the generic concept, andtherefore, such adaptations and modifications should and are intended tobe comprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology employed herein is for the purpose of description and not oflimitation. The means, materials, and steps for carrying out variousdisclosed functions may take a variety of forms without departing fromthe invention.

Thus, the expressions “means to . . . ” and “means for . . . ”, or anymethod step language, as may be found in the specification above and/orin the claims below, followed by a functional statement, are intended todefine and cover whatever structural, physical, chemical, or electricalelement or structure, or whatever method step, which may now or in thefuture exist which carries out the recited functions, whether or notprecisely equivalent to the embodiment or embodiments disclosed in thespecification above, i.e., other means or steps for carrying out thesame function can be used; and it is intended that such expressions begiven their broadest interpretation.

1. Apparatus for measuring blood pressure, comprising a generallytubular constrictable sleeve or cuff for a limb of a person, a sourcefor fluid pressure, means for measuring static pressure, and microphonemeans arranged in proximity to an artery, characterised in that the cuffis at least partly enclosed in two essentially concave shell partsdisplaying a stiffness along the limb, said shell part being openableagainst a restoring force, and in that a linear array of microphoneelements is disposed on a universal joint type support in one shell partessentially perpendicular to the longitudinal axis of such shell partand near the lower end.
 2. Apparatus according to claim 1, characterisedin that signal selection means of the diversity reception type are usedto select the microphone that provides the best signal-to-noise ratio.3. Apparatus according to claim 1 , characterised in that the microphonesignal is amplified and made available to an electroacoustic converterfor enabling listening to the signal.
 4. Apparatus according to claim 3,characterised in that the signal is output via a built-in speaker in theapparatus.
 5. Apparatus according to claim 3, characterised in that thesignal is output via a wireless link to a receiver connected toearpieces carried by an auscultating physician.
 6. Apparatus accordingto claim 1, characterised in that it comprises signal processing meansfor combining information derived from measurements of slowly varyingstatic pressures with information from said microphone means in order toobtain a numerical value for a blood pressure.
 7. Apparatus according toclaim 1, characterised in that an inelastic strap attached to one shellpart is provided to close the gap between the shell parts.
 8. Apparatusaccording to claim 7, characterised in that the strap is provided withmeans locking to the other shell part in conjunction with theoverlapping of said strap and said other shell part.
 9. Apparatusaccording to claim 8, characterised in that the amount of overlapbetween the strap and the shell part is used as a circumference measurefor correcting the reading of blood pressure.
 10. An apparatus accordingto claim 9, characterised in that the overlap is measured capacitivelybetween an electrode or a plurality of electrodes fixed to the cuff andan electrode or a plurality of electrodes fixed to the strap. 11.Apparatus according to claim 1, characterised in that the shells arefitted on hinge parts connected to handle parts operable by one hand.12. Apparatus according to claim 1, characterised in that in addition toa stiffness in the longitudinal direction the shell structure displays aresilience in the circumferential direction.
 13. Apparatus according toclaim 12, characterised in that the hinge is a continuous resilient partjoining the shell parts.
 14. Apparatus according to claim 12,characterised in that the shell parts are integral with the hinge part,forming one continuous sheet of material.
 15. Apparatus according toclaim 14, characterised in that the continuous sheet of material assumesa generally frusto-conical shape in its closed state.
 16. Apparatusaccording to claim 11, characterised in that mechanical actuating meansfitted in proximity to the hinge part compress one shell part towardsthe other during measurement.
 17. Apparatus according to claim 16,characterised in that the mechanical actuating means consist of an aircylinder and levers.
 18. Apparatus according to claim 16, characterisedin that the mechanical actuating means consist of strings fitted nearthe inner side of each shell part and disposed perpendicular to thelongitudinal axis of such shell part.
 19. Apparatus according to claim1, characterised in that an inflatable cuff forms an inner lining to theshell parts, providing an inflatable main air chamber.
 20. Apparatusaccording to claim 7, characterised In that an inflatable cuff forms aninner lining to the shell parts, providing an inflatable main airchamber, that the strap is provided with air chambers disposedessentially perpendicular to the orientation of the shells andcommunicating with the main air chamber.
 21. Apparatus according toclaim 1, characterised in that the universal joint is emulated by a foampad.
 22. Apparatus according to claim 1, characterised in that theuniversal joint is emulated by means of a separate air chamber fittedbetween the cuff and the microphone array.
 23. (canceled)
 24. (canceled)25. Apparatus according to claim 1, wherein said sleeve or cuff isadapted to fit an arm of a person.
 26. Apparatus according to claim 1,wherein said sleeve or cuff is adapted to fit an leg of a person.